Abstract: :
Purpose: The incidence of squamous cell and basal cell carcinomaof the lower lid is higher than the incidence on the upper lid.A possible explanation is increased exposure to ultravioletradiation of the lower lid margin from reflected ultravioletlight from the tear film overlying the cornea. Using principlesof optical reflection, the theoretical amount of additionalradiative flux can be calculated.Methods: A mathematical modelwas developed using normal values of corneal curvature and basicprinciples of reflectivity at an air-water interface. An idealspherical surface of the cornea was assumed with the lower lidmargin at the base of the cornea. Fresnel’s law of reflectionwas used to calculate the amount of reflected light from thecornea at different incident angles of sunlight. All incomingrays were assumed to be parallel. Only the reflection off theanterior tear film surface was considered.Results: Preliminarydata demonstrate a maximal increase in radiation intensity onthe lower lid when the angle of incident light is 72.9 degreesabove the horizon. This result applies only to the portion ofthe lower lid directly below the most anterior aspect of thecornea. Most of the reflected rays come from 1.4 to 2.2 mm belowthe apex of the cornea, where a glancing angle causes the majorityof the light to be reflected instead of transmitted. The quantityof additional flux depends on assumptions of lid width and thenumber of discrete computational corneal segments used in themodel, but appears to be at least 50% over the normal incidentflux.Conclusion: In theory, there is increased radiative fluxto the lower lid from reflected ultraviolet light from the cornea.The higher incidence of carcinoma on the lower lid may stem,in part, from the higher exposure to ultraviolet radiation.However, the amount of reflected ultraviolet light may not beenough to explain the difference in incidence of carcinoma betweenthe lower lid and upper lid. Confirmation of the theoreticalresults can be obtained by laboratory measurements using a simulatedcorneal surface or by real measurements on human lids.